This invention relates to a package for semiconductor diodes and, in particular, to button diodes in which the semiconductor die has a non-planar surface.
The button diode is a particularly useful package configuration for semiconductor rectifiers and has achieved widespread commercial use. An example of this type of package is disclosed in U.S. Pat. No. 4,093,958. The improved, low-cost package described therein comprises a semiconductor diode chip or die soldered between two contact members, hereinafter called studs, having good thermal conductivity. The studs have a cross-section slightly larger than the semiconductor die. A planar face on each stud is soldered, respectively, to the planar and substantially parallel faces of the chip. The diode chip and the ends of the stud to which it is soldered are surrounded with a suitable encapsulant which protects the chip and insulates the studs from each other. Because the major faces of the semiconductor chip are planar, the solder provides substantially uniform contact between the stud and semiconductor device across the whole area of the semiconductor die irrespective of its shape, whether round, square, or rectangular.
There are a number of semiconductor devices, Schottky rectifiers for example, wherein at least one of the major faces is not planar, even though they may be referred to as using planar passivation techniques. In these devices there is typically present on the surface of the semiconductor die a protruding passivation region (typically a dielectric oxide or equivalent) surrounding the junction periphery, often in the shape of an annular ring near the edges of the chip. Thus, the surface of the semiconductor to be contacted is in the bottom of a moat whose sidewalls are formed by the passivation region. This type of depressed surface is not readily contacted by a planar stud. Thus, in the prior art, the stud is provided with a boss or raised portion on the surface to be mated to the semiconductor chip. Since the studs have cylindrical symmetry, the boss usually also has cylindrical symmetry. Alternatively, a separate disk may substitute for the boss and be soldered to the stud at the same time as the die is attached.
The circular boss or disk of the prior art provides a poor match to the rectangular semiconductor die typically utilized. The circular boss contacts less than 80% of available contact area on a rectangular chip which, in turn, increases the series resistance, and degrades the power handling capabilities of the device. Thus, a need continues to exist for a button diode design which overcomes the difficulties described above inherent in the use of non-planar die.
Accordingly, it is an object of the present invention to provide an improved package for semiconductor power diodes with non-planar surfaces.
A further object of the present invention is to provide an improved package for semiconductor diodes with rectangular non-planar die.
Another object of this invention is to make possible the use of identical stud members for assembly to both planar and non-planar surfaces of a die.
A further object of the present invention is to provide improved transient response for semiconductor packages using non-planar rectangular die.
It is an additional object of the present invention to provide a process whereby an improved package for semiconductor diodes using non-planar die may be assembled.